U.S. radiation-safety experts said that, based on radioactivity levels detected in Tokyo's tap water, health risks for most people generally were slight.

Government officials in Tokyo urged special precautions with drinking water after detecting traces of radioactive iodine-131 in the water supply that were twice the acceptable level of exposure for children. The material was below the government's exposure limits for adults.

Radioactive iodine is especially worrisome to pregnant women and children because the body naturally concentrates the isotope in the thyroid gland, where it quickly can affect growth.

"The reason that iodine-131 is so dangerous in children is that their normal growth and development, especially of the brain, depends on the thyroid gland," said University of Buffalo Medical School neurologist Alan Lockwood, who sits on the board of Physicians for Social Responsibility, a nonprofit group that advocates against nuclear proliferation. "And if there is exposure as a child, the risk of developing thyroid cancer later in life is higher."

Generally, all water has a natural background level of radiation that varies from place to place.

The amount of contaminated food or water someone would have to consume to reach a dangerous dose would vary by the level of radioactivity, the type of element ingested and the length of time over which it was consumed. Iodine-131 has a relatively short half life—its radioactivity drops by half in eight days—meaning it naturally becomes safer somewhat quickly. The half life of cesium-137, in contrast, is 30 years.

At the highest levels of radiation detected in Tokyo's water, a normal adult would have to drink about 52 gallons of contaminated water to reach the annual U.S. limit for radiation exposure, according to Peter Caracappa, a clinical professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, N.Y.

Depending on the other factors that affect any individual's risk of cancer, the current level of exposure measured in Tokyo's water might raise the risk of thyroid cancer by four one-thousandth of a percent, Dr. Caracappa said. "If I needed to drink water, at the current levels, I would drink it," he said. "It is human nature to have an abundance of caution when it comes to children."

Corrections & Amplifications:

An earlier version of this article incorrectly said that neurologist Alan Lockwood works for the University of Pittsburgh.

This copy is for your personal, non-commercial use only. Distribution and use of this material are governed by our Subscriber Agreement and by copyright law. For non-personal use or to order multiple copies, please contact Dow Jones Reprints at 1-800-843-0008 or visit www.djreprints.com.